Where do you get the estimate of "once or twice per galaxy"? I can agree that it might be "rare," but I have no idea how you would pin down the rareness to that level of accuracy.
It's a total guesstimate. The interesting thing to me is that I have no idea how anyone else pins down THEIR estimates to any level of accuracy, but they are apparently confident enough in their estimates to posit things like the "Great Filter", when other (equally likely) estimates such as mine require no such thing.
